In the widely used commercial process for the manufacture of cement, the steps of drying, calcining, and clinkering cement raw materials are accomplished by passing finely divided raw materials, including calcareous minerals, silica and alumina, through a heated, inclined rotary vessel or kiln. In what is known as conventional long dry or wet process kilns the entire mineral heating process is conducted in a heated rotating kiln cylinder, commonly referred to as a “rotary vessel.” The rotary vessel is typically 10 feet to greater than 25 feet in diameter and 150-700 feet in length (with typical length-to-diameter ratios being anywhere from 15:1 to 40:1) and is inclined so that as the vessel is rotated, raw materials fed into the upper end of the kiln cylinder move under the influence of gravity toward the lower “fired” end where the final clinkering process takes place and where the product cement clinker is discharged for cooling and subsequent processing. Kiln gas temperatures in the fired clinkering zone of the kiln range from about 1300° C. (˜2400° F.) to about 2200° C. (˜4000° F.). Kiln gas exit temperatures are as low as about 250° C. (˜400° F.) to 350° C. (˜650° F.) at the upper mineral receiving end of so-called wet process kilns. Up to 1100° C. (˜2000° F.) kiln gas temperatures exist in the upper end of dry process rotary kilns.
Generally, skilled practitioners consider the cement making process within the rotary kiln to occur in several stages as the raw material flows from the cooler gas exit mineral feed end to the fired/clinker exit lower end of the rotary kiln vessel. As the mineral material moves down the length of the kiln it is subjected to increasing kiln gas temperatures. Thus in the upper portion of the kiln cylinder where the kiln gas temperatures are the lowest, the in-process mineral materials first undergo a drying/preheating process and thereafter move down the kiln cylinder until the temperature is raised to calcining temperature. The length of the kiln where the mineral is undergoing a calcining process (releasing carbon dioxide) is designated the calcining zone. The in-process mineral finally moves down the kiln into a zone where gas temperatures are the hottest, the clinkering zone at the fired lower end of the kiln cylinder. The kiln gas stream flows counter to the flow of in-process mineral materials from the clinkering zone, through the intermediate calcining zone and the mineral drying/preheating zone and out the upper gas exit end of the kiln into a kiln dust collection system. The flow of kiln gases through the kiln can be controlled to some extent by a draft induction fan positioned in the kiln gas exhaust stream. Over the last 10-20 years preheater/precalciner cement kilns have proven more energy efficient than the traditional long kilns. In preheater/precalciner kilns the raw mineral feed is heated to calcining temperatures in a stationary counterflow preheater/precalciner vessel before it drops into a heated rotary vessel for the higher temperature clinkering reactions.
Some kiln operators have experimented with selective non-catalytic reduction (SNCR) as a method to reduce nitrogen oxide (NOX) emissions. SNCR has shown to be an effective and retrofittable NOX control technique, as discussed in “A Mode for Prediction of Selective Noncatalytic Reduction of Nitrogen Oxides by Ammonia, Urea, and Cyanuric Acid with Mixing Limitations in the Presence of CO” by Brower et al., Twenty Sixth Symposium (International) on Combustion/The Combustion Institute, 1996, pp. 2117-2124, the entirety of which is hereby incorporated by reference. SNCR has been demonstrated in cement kilns where a continuous stream of urea or ammonia can be introduced into cement kilns in the critical temperature region where the SNCR reaction takes place, 900° C. to 1100° C. In preheater/precalciner cement kilns the critical temperature zone is in the stationary portion of the preheater/precalciner, downstream of the rotary kiln, where it is practical to introduce a continuous stream of ammonia or urea solution across the gas stream. In conventional long process cement kilns the exhaust gas temperature is typically less than 600° C., well below the minimum 900° C. required for the SNCR reaction to occur. In some long kilns, concepts have been suggested to inject urea from the gas discharge end of the kiln under high velocity in order to reach the necessary temperature zone, such as described U.S. Pat. No. 5,728,357. However, to the extent that such a method is even effective, it is not practical for kilns where it is not possible to inject the urea to the critical temperature zone from the end of the kiln due to the existence of internal heat exchange apparatus like a chain system or the distance is simply too far (i.e., over 50 meters).
As such, it has also been attempted to introduce urea through an opening in the wall of the rotating kiln. For example, urea prills have been introduced through an opening in the kiln wall, such as through the tire drop tube. However, no significant response (i.e., NOX reduction) was observed. This is not surprising even when the urea addition point is in the correct temperature range. This is true for a number of reasons. For example, the opportunity for the introduction of urea through an opening in the kiln wall only happens once per revolution in the current tire injection drop tubes. Long dry kilns typically rotate once every 45 seconds. The gas velocity where the temperatures are between 900° C.-1100° C. is about 6 to 10 meters per second. The total gas residence time in the critical temperature range for SNCR to occur is in the range of about 3 seconds. Urea is commercially available primarily in the form of prills of 1 mm to 2 mm diameter since the primary use is as a fertilizer (prills facilitate spreading) or for dissolving into a water solution such as used for aqueous injection of urea for SNCR (where prills facilitate dissolution). The addition of the prills into a gas stream of 900°-1100° C. results in almost instantaneous volatilization of the urea (which has a dissociation temperature of 133° C.) because of the high surface area exposed to transfer heat from the kiln gas or from the mineral bed at 800° C. in which it may come in contact. Therefore, a charge of urea prills treats the kiln gas for only a small portion of time between charges, probably at most for only one or two seconds. Thus, in the 45 seconds between charges, there is only a few seconds where volatiles are being released from the urea and the majority of kiln gasses miss the treatment.
An additional problem to be overcome in the implementation of SNCR is the stratification of the kiln gasses. In the zone of the kiln where the gas temperature is 900°-1100° C., the material temperature at the bottom of the kiln is at the calcining temperature of 850° C. and is liberating CO2 at a molecular weight of 44 vs. 30 for the kiln gas. Because of the gas density difference, the gasses at the bottom of the kiln stay at the bottom so there is a large temperature difference between the gasses at the bottom and those at the top of the kiln. Further, the added urea will fall to the mineral bed at the bottom of the kiln where it will release its volatiles. These volatiles will tend to stay at the bottom of the kiln and not treat the full gas cross section resulting in slipping by of the gasses at the top of the kiln which will leave the kiln untreated. Typically, long kilns are 4 to 6 meters in diameter and the gas velocities are 6 to 10 meters per second.
Typically, preheater/precalciner kilns utilizing SNCR use aqueous ammonia or aqueous urea. Aqueous ammonia generally cost about $700 per ton of ammonia. Anhydrous ammonia (ammonia gas) is significantly less expensive at $400 per ton. However, the more cost effective anhydrous ammonia is not generally used for a number of reasons. Firstly, anhydrous ammonia must be handled as a hazardous material. This involves certain regulatory reporting requirements and the like. Moreover, anhydrous ammonia is difficult to mix into the entire cross section of the kiln gasses in the kiln duct.